In a twin roll caster, molten metal is introduced between a pair of counter-rotated horizontal casting rolls that are cooled so that metal shells solidify on the moving roll surfaces and are brought together at a nip between them to produce a solidified strip product delivered downwardly from the nip between the rolls. The term “nip” is used herein to refer to the general region at which the rolls are closest together. The molten metal may be poured from a ladle into a smaller vessel or series of smaller vessels from which it flows through a metal delivery nozzle located above the nip forming a casting pool of molten metal supported on the casting surfaces of the rolls immediately above the nip and extending along the length of the nip. This casting pool is usually confined between side plates or dams held in sliding engagement with end surfaces of the rolls so as to dam the two ends of the casting pool against outflow.
The twin roll caster is capable of continuously producing cast strip from molten steel through a sequence of ladles positioned on a turret. Pouring the molten metal from the ladle into a tundish and then a moveable tundish before flowing through the metal delivery nozzle enables the exchange of an empty ladle for a full ladle on the turret without disrupting the production of the cast strip.
In casting thin strip by twin roll caster, the crown of the casting surfaces of the casting rolls varies during a casting campaign. The crown of the casting surfaces of the casting rolls in turn determines the strip thickness profile, i.e., cross-sectional shape, of the thin cast strip produced by the twin roll caster. Casting rolls with convex (i.e. positive crown) casting surfaces produce cast strip with a negative (i.e. depressed) cross-sectional shape; and casting rolls with concave (i.e. negative crown) casting surfaces produce cast strip with a positive (i.e. raised) cross-sectional shape. The casting rolls generally are formed of copper or copper alloy, usually coated with chromium or nickel, with internal passages for circulation of cooling water enabling high heat fluxes for rapid solidification where the casting rolls undergo substantial thermal deformation with exposure to the molten metal during a casting campaign.
In thin strip casting, a roll crown is desired to produce a desired strip cross-sectional thickness profile under typical casting conditions. It is usual to machine the casting rolls when cold with an initial crown based on the projected crown in the casting surfaces of the casting rolls during casting. However, the differences between the shape of the casting surfaces of the casting rolls between cold and casting conditions are difficult to predict. Moreover, the crown of the casting surfaces of the casting rolls during the casting campaign can vary significantly. The crown of the casting surfaces of the casting rolls can change during casting due to changes in the temperature of the molten metal supplied to the casting pool of the caster, changes in casting speed of the casting rolls, and other casting conditions, such as slight changes in molten steel composition.
Previous proposals for casting roll crown control have relied on mechanical devices to physically deform the casting roll; for example, by the movement of deforming pistons or other elements within the casting roll or by applying bending forces to the support shafts of the casting rolls. However, these previous proposals for casting roll crown control have limitations. For example, Japanese Patent No. 2544459 (herein “JP '459”) describes a casting roll with internal “water-cooled roll heating means embedded in the two end parts” used to control the deformation experienced at each roll end during casting. See, JP '459, Section: “Means employed in order to solve the problem”. The casting rolls are solid metal rolls with internal cooling channels, which require water heating means at the end of the casting rolls. The limitations of the caster disclosed in JP '459 are discussed in U.S. Pat. No. 5,560,421 (herein “the '421 Patent”), which states that “the thermal capacitance of each drum 01 to be heated is large, a deformation responsibility of the shape of the outer surfaces of the drum to be controlled is low and it would be difficult or impossible to timely control the workpiece”. Patent '421, col. 1, 11, 64-col. 2, 11, 1. The '421 Patent continues to explain, “it would be impossible to suitably control the shape of the workpiece to be continuously cast”. Id., col. 2, 11, 6-7. The '421 Patent proposes a solution in which the solid casting rolls have end cutouts with large external (to the solid roll) annular elements heated by water. These annular elements are used to change the profile of the casting roll.
However, large solid casting rolls such as those proposed by JP '459 and the '421 Patent are expensive to manufacture, have shorter service life (due to the effects of thermal fatigue from the cyclic heat flux experienced during twin roll casting on larger cylinder masses), and are much less responsive due to their large thermal mass.